This invention relates to a brake apparatus having a first wall that pushes a pressure responsive device while a second wall simultaneously pulls the first wall in response to the development of a pressure differential across the first and second walls.
Tandem servomotors of which that disclosed in U.S. Pat. No. 3,517,588 is considered typical, have first and second walls that are integrally tied to an output member that passes through a stationary wall that divides a housing into substantially equal first and second operational units. This type tandem servomotor is adequate for most operations. However, under some conditions, axial deflection can occur since all reaction forces must be carried through the housing to a stationary mounting structure. Such axial deflection can result in increased brake pedal travel in order to achieve a desired braking force. In order to reduce axial deflection, such a servomotor housing must be constructed of material that can resist bending. As a result, the thickness of such material from which the housing is constructed increases the weight of the servomotor as compared to the thickness required for only the operation of the servomotor.
In an effort to reduce the weight of a servomotor it has been proposed in U.S. Pat. No. 4,270,353 that the housing could be constructed of a lightweight material if the reaction forces were carried from a master cylinder to a stationary firewall by tie rods. In such structure, the movable wall must be sealed with respect to the tie rods which can result in frictional resistance under some operating conditions.
In an effort to avoid additional seals on the movable wall it has been proposed in U.K. Pat. No. 2,040,376 that the reaction forces be carried through a tube passing through the center of the servomotor housing. The tube is connected to the master cylinder and the stationary firewall of the vehicle. The movable wall is sealed to the tube and as a result that effective area is removed from the development of an output force.